Head Disk Assembly
<hardware, storage> (HDA) A sealed, high capacity mainframe hard disk
with integral heads, as opposed to a removable disk.
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1. The portion of a packet, preceding the actual data, containing source and
destination addresses, error checking and other fields.
2. The part of an electronic mail message or news article that precedes the body
of a message and contains, among other things, the sender's name and e-mail
address and the date and time the message was sent.
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Head Normal Form
<theory, reduction> (HNF) A term describing a lambda expression whose top
level is either a variable, a data value, a built-in function applied to too few
arguments, or a lambda abstraction whose body is not reducible. I.e. the top
level is neither a redex nor a lambda abstraction with a reducible body.
An expression in HNF may contain redexes in argument postions whereas a normal
form may not.
Compare Weak Head Normal Form.
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head normalisation theorem
Under the typed lambda-calculus, beta/delta reduction of the left-most redex
(normal order reduction) is guaranteed to terminate with a head normal form if
one exists. See also Church-Rosser theorem.
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head-strict » heap
[Sun] Concentrating, usually so heavily and for so long that everything outside
the focus area is missed. See also hack mode and larval stage, although this
mode is hardly confined to fledgling hackers.
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<theory> A head-strict function will not necessarily evaluate every cons
cell of its (list) argument, but whenever it does evaluate a cons cell it will
also evaluate the element in the head of that cell. An example of a head-strict
beforeZero :: [Int] -> [Int]
beforeZero  = 
beforeZero (0:xs) = 
beforeZero (x:xs) = x : beforeZero xs
which returns a list up to the first zero.
This pattern of evaluation is important because it is common in functions which
operate on a list of inputs.
See also tail-strict, hyperstrict.
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